[x264-devel] x264 encoding corruption bug w/ AltiVec enabled

[Timothy Pearson]

When attempting to use the latest libx264 with the latest ffmpeg sources
on Talos II (ppc64le), a color / blocking artifact is introduced in the
source video.  These artifacts disappear entirely when libx264 is
rebuilt with asm support (AltiVec) disabled, but the machine is then no
longer able to maintain 60fps framerate on the sample live stream.

The artifacts vary based on preset:

ultrafast: what looks like random pixel noise, it slowly fades in over
the source video then suddenly disappears, repeating every second or so.

veryfast: screen is entirely corrupted in a semi-regular grid of highly
saturated colors

fast: moving rainbow colored blobs over the aforementioned grid

A similar form of corruption is present in the Debian package version of
libx246, sharing the odd "fading in / reset / fading in" characteristics
of the corruption seen in the latest GIT master.  The Debian packaged
versions almost look like the saturation is being cranked up then
suddenly reset every second or so.

I can provide access to hardware for testing / development if needed.
In general I have also not noted much speedup for the AltiVec routines
especially on ultrafast, I'm not sure if this is related to the
corruption or not at this point.

If you need test video I can try to run an unrestricted source through
the encoder, but I suspect the artifacts will be quite apparent no
matter what source you use for testing / debugging.

Thanks!

-- 
Timothy Pearson
Raptor Engineering
+1 (415) 727-8645 (direct line)
+1 (512) 690-0200 (switchboard)
https://www.raptorengineering.com